Electrical slip rings are used to transfer electrical power and/or signal(s) between a rotor and a stator. These devices are used in many different military and commercial applications, such as solar array drive mechanisms, aircraft and missile guidance platforms, undersea robots, CATSCAN systems, and the like. In some of these applications, slip rings are used in conjunction with other rotary components, such as torque motors, resolvers and encoders. Electrical slip rings must be designed to be located either on the platform axis of rotation, or be designed with an open bore which locates the electrical contacts off-axis. Hence, the designations “on-axis” and “off-axis” slip rings, respectively. The diameter of slip ring motors may range from a fraction of an inch to multiple feet, and the angular speed may vary from one revolution per day to as much as 20,000 revolutions per minute. In all of these applications, the electrical contacts between the rotor and stator must: (1) transfer power and/or signal(s) without interruption at high surface speeds, (2) have long wear life, (3) maintain low electrical noise, and (4) be of a physical size that allows multiple circuits to be packaged in a minimum volume.
The most efficient management of the electrical and mechanical contact physics allows the most demanding requirements to be met. For example, if the application is an off-axis slip ring that allows the x-ray tube in a CATSCAN gantry to rotate about the patient's body, the electrical contacts must be designed to carry about 100-200 amps (with surges of hundreds of amps), operate at surface speeds on the order of 500 inches per second, last for 100 million revolutions, and occupy a minimal volume within the gantry. In order to meet the 100 million revolution requirement for a device that is about six feet in diameter, the brush force must be low to minimize frictional heating and to maintain a large number of contact points between brush and ring to achieve the required current density.
Four types of electrical contacts between a rotor and stator include: (1) a composite solid material brush on a cantilevered spring, (2) a monofilament metal alloy brush that tangentially engages the rotor, (3) a fiber brush having a plurality of individual fibers, with the bundle tangentially engaging the rotor, and (4) a tip-of-fiber contact between the brush and rotor. The contact force, surface speeds and type of lubrication for each contact type is summarized in Table I. Table I also shows the types of lubricants required to reduce the contact frictional heating if the brush force is above one gram.
TABLE IType ofContact TypeContact ForceSurface SpeedsLubricationcomposite brush0.4kg/cm2700 in/secsacrificialgraphite film*monofilament3-20grams12 in/secboundarymetal alloylubrication**tangential fiber1-3grams200 in/secadventitious***brushfiber-on-tip0.1-1grams1200 in/secadventitious****With a sacrificial graphite film, the brush and ring interface is lubricated by a film of graphite that is transferred from the brush to the ring. Material that is worn away is replaced by graphite from the brush.**With boundary lubrication, a boundary lubricant film supports a portion of the load between the contact members. The points of metal contact support the remaining load between the contact members, and provide the current-carrying capability.***With adventitious films, very thin films of materials that are capable of reducing the coefficient of friction between the contact members under light loads.
The tribological properties of electrical contacts and the right choice of lubricant to meet the requirements of the application are extremely important. For example, if the contacts are to be used in a space application, then the lubricant must meet all of the requirements of a ground based application, and have a low vapor pressure. If the contacts have a long life requirement, then dust, wear debris and other contaminants may accumulate in the contact zone and create problems with life and signal transfer. However, if the electrical contact members can be brought together with a force of about one gram or less, then the lubricant and the associated complications are eliminated.
For several years, fiber brushes with a tangential orientation to the ring have been successfully used to meet high surface speeds without the use of a lubricant.
When manufacturing slip rings in the range of four to six feet in diameter, the costs of the ring material, as well as the costs associated with the equipment used to cast the dielectric material that supports the rings, the costs of equipment required to machine the support structure, and the costs of the equipment used to electroplate precious metal on a ring, rise dramatically if a continuous ring approach is used. Large-diameter rings are normally machined from plate stock or tubing of the appropriate size. Another option is to form a metal strip of the required cross-section, to bend it into an annulus or ring, and to weld the facing ends together. In this case, the dimensional tolerances that must be held for the ring I.D. and O.D. cause the continuous ring to be prohibitively expensive. In addition, the bath required to electrodeposit metal on a six foot diameter ring is five to six times more expensive than that required for a 120° length of arc used to fabricate a segmented slip ring of the same diameter.
U.S. Pat. No. 5,054,189, the aggregate disclosure of which is hereby incorporated by reference, teaches a method of manufacturing an annular dielectric base portion of an electrical slip ring assembly having multiple electrical rings formed in the outer circumference. The rings are formed from conductive metal strips of the appropriate cross-sectional shape and configuration. When each ring is wrapped around the circumference of the base, the facing ends are intended to abut one another. However, because of dimensional variations in the base O.D. and dimensional variations in the length of the strip used to form the conductive ring, the facing ring ends sometimes do not abut properly. In practice, the length of the ring is controlled such that a gap always exists between the facing ring ends. This gap may vary from about 0.020 inches to about 0.040 inches. The brush technology used with this ring structure is the tangential fiber brush, which can readily move over that gap without mechanical and/or electrical interference. Over ten years of experience has shown that as the slip ring rotates, brush and ring wear debris and other particulate contaminants will accumulate in the gap. As the brushes continue to move over the gap, finely divided particles are dragged onto the ring surface, creating electrically-insulating films. Thus, problems develop with electrical signal transmission. Millions of ring revolutions may occur because these problems develop.
It would be generally desirable to provide an improved compact slip ring that would allow longer life, higher current densities, and higher rotor surface speeds to be achieved a lower costs that with current slip ring technology, and that uses fiber-on-tips electrical contact technology.